There have been proposed plate-making methods of a lithographic plate using a computer. In particular, in a CTP system, plate making is performed by printing print image information edited and produced by way of DTP (desktop publishing) directly on a plate material without any imaging processing, using a laser or thermal head. This CTP system is extremely prospective in the field of commercial printing because the system will enable rationalization of plate making process, reduction in time needed for plate making, and reduction in material cost.
In regard to plate materials for use in such a CTP system, the present applicants propose a thermosensitive plate material which has a printing surface (surface on which ink is put at the time of printing) on which an ink receiving portion and an ink non-receiving portion are formed by writing with heat according to the print image information, and which requires no development process and provides a lithographic plate excellent in printing resistance. This plate material is called “thermosensitive plate material for lithographic plate formation”.
The lithographic plates obtained by the plate making with this plate material are used for, for example, printing using oil-based ink, and, on the printing surface, an oil-based ink receiving portion (lipophilic portion) and an oil-based ink non-receiving portion (hydrophilic portion) are formed at the time of plate making. Upon printing, the ink is retained in the lipophilic portion on the printing surface, and in the offset printing, an image corresponding to the lipophilic portion on the printing surface is formed on paper by pressing the ink against the paper via a rubber blanket.
For example, JP 7-1849 A discloses a thermosensitive material for use in plate material which contains a micro-capsule with a component (lipophilic component) being to form an lipophilic portion (image portion) by heat and a hydrophilic polymer (hydrophilic binder polymer). The hydrophilic polymer has a functional group capable of three-dimensionally crosslinking and a functional group reacting and chemically bonding with the lipophilic component in the micro-capsule after fracture of the micro-capsule resulting from application of heat.
The publication also discloses a plate material produced by forming a thermosensitive layer (hydrophilic layer) composed of the above described thermosensitive material on the surface of a support and then three-dimensionally crosslinking the hydrophilic polymer. According to the publication, this plate material is constructed in such a manner that the lipophilic component in the micro-capsule forms a polymer and becomes an lipophilic portion (image portion) once the micro-capsule is fractured by heat during plate-making, and at the same time, the lipophilic component reacts and chemically bonds with the hydrophilic polymer.
As a result, the plate material does not require the development process in the plate making operation, and the lithographic plates obtained are markedly excellent not only in printing resistance but also in the performance of the hydrophilic portion (non-image portion), whereby clear printed matter free from scumming (slight smears which are uniformly formed) can be obtained.
In addition, WO (international publication) 98/29258 discloses a method of further enhancing the printing resistance of the plate materials described in JP 7-1849 A in which three-dimensional crosslinking of the hydrophilic polymer is generated by allowing Lewis base moieties containing nitrogen, oxygen, or sulfur and polyvalent metal ions, such as tin, to interact with each other.
The publication also describes a method of stabilizing the hydrophilic portion (non-image portion) on the printing surface as well as preventing dirt from adhering to the printing surface by forming a hydrophilic polymer thin film layer, as a protective agent for the surface, on the surface of a thermosensitive layer (hydrophilic layer).
With the plate materials described in the above publications, the lithographic plates which do not require the development process and are excellent in printing resistance as well as in performance of the hydrophilic portion (oil-based ink non-receiving portion) can be obtained, as described above. These plate materials, however, leave much to be improved in terms of mechanical strength and printing performance (in particular, preventing dirt in the portion where an image of a printed matter is not formed (non-image portion)) of the lithographic plate obtained by the plate making.
In contrast, WO 00/63026 discloses that the mechanical strength and printing performance of a lithographic plate are enhanced, which is obtained by allowing a polyvalent metal oxide or molecules having bonds represented by (SiO2)n to be contained in a thermosensitive layer of a thermosensitive plate material for lithographic plate formation and subjecting the plate material to plate-making. However, this plate material can also be further improved in terms of the printing performance (in particular, a non-image portion is unlikely to be contaminated) of printed matter by a lithographic plate obtained by plate-making.
On the other hand, JP 2000-25353 A describes that a porous configuration with an average pore diameter of 0.05 to 1 μm is formed on the surface of a hydrophilic layer containing a lipophilic component and a hydrophilic binder polymer that are micro-capsulated, which is a thermosensitive layer of a thermosensitive plate material for lithographic plate formation. Furthermore, it is described that if a lithographic plate obtained by subjecting the plate material to plate-making is used, special dampening water is not required for printing, and the amount of dampening water to be used can be minimized.
However, in the plate material described in the above publication, micro-capsules are present on a surface side of the thermosensitive layer (for example, in a portion within 0.1 μm from the surface). Therefore, the micro-capsules are likely to be exposed to the surface of a lithographic plate obtained by subjecting the plate material to plate-making during printing. Therefore, in the case where the surfaces of the micro-capsules do not have sufficient hydrophilicity, oil-based ink adheres to the exposed micro-capsules, which may cause scumming in the non-image portion of the printed matter.
JP 2001-18547 A describes that a printing plate excellent in hydrophilicity, water resistance, and printing resistance is obtained by making the surface of a hydrophilic layer mainly made of an organic substance porous. However, when a porous configuration mainly made of the organic substance is present on the surface of the printing plate, the mechanical strength required for the printing plate is difficult to obtain.
JP 2001-30645 A describes that, as a thermosensitive layer of a thermosensitive plate material for lithographic plate formation, a layer is formed in which composite particles at least composed of a hydrophobic precursor and a photothermal conversion agent are dispersed in a hydrophilic medium. In this plate material, by using a sol-gel converting material as the medium, a high printing performance is obtained. Furthermore, JP 2001-30645 A describes that resin having a siloxane bond and a silanol group is preferable as the medium.
Furthermore, WO98/40212 and WO98/40213 describe a plate material having a specific lipophilic layer and lipophobic layer on a substrate, which can be produced easily at a low cost without a development process.
In the plate materials described in these publications, the lipophilic layer is formed on the substrate, and the lipophobic layer is formed thereon. The lipophobic layer is composed of a colloid made of a specific metal oxide or metal hydroxide, and a matrix made of a cross-linking polymer. In the plate materials described in these publications, the matrix made of the cross-linking polymer is considered to be formed by sol-gel conversion and dehydration and condensation of a silane coupling agent.
However, the elasticity of the layer formed by the sol-gel conversion and the dehydration and condensation of the silane coupling agent is not sufficient for a printing plate.
JP 11-334239 A describes that a plate material to be subjected to plate-making by ablation includes a photosensitive layer and a hydrophilic layer formed on a substrate in this order, and fine particles of titanium oxide and/or zinc oxide are contained in the hydrophilic layer so as to enhance a removal efficiency of the hydrophilic layer.
However, this plate material has problems in that substances scattering during ablation may contaminate an optical system to be used for ablation and adhere to an obtained plate.
A first object of the present invention is to provide a thermosensitive plate material for lithographic plate formation requiring no development process, in which a printing performance (in particular, a non-image portion is unlikely to be contaminated) of printed matter by a lithographic plate obtained by subjecting the plate material to plate-making is enhanced, and which has mechanical strength required for a printing plate.
A second object of the present invention is to enhance a water-retention capacity of a lithographic plate obtained by plate-making and reduce an amount of dampening water to be used during printing, while achieving the above-mentioned first object.